Revert "sched: Remove sched_ktime_clock()"

This reverts 'commit 24c18127e9ba ("sched: Remove sched_ktime_clock()")'

WALT accounting uses ktime_get() as time source to keep windows in
align with the tick. ktime_get() API should not be called while the
timekeeping subsystem is suspended during the system suspend. The
code before the reverted patch has a wrapper around ktime_get() to
avoid calling ktime_get() when timekeeping subsystem is suspended.

The reverted patch removed this wrapper with the assumption that there
will not be any scheduler activity while timekeeping subsystem is
suspended. The timekeeping subsystem is resumed very early even before
non-boot CPUs are brought online. However it is possible that tasks
can wake up from the idle notifiers which gets called before timekeeping
subsystem is resumed.

When this happens, the time read from ktime_get() will not be consistent.
We see a jump from the values that would be returned later when timekeeping
subsystem is resumed. The rq->window_start update happens with incorrect
time. This rq->window_start becomes inconsistent with the rest of the
CPUs's rq->window_start and wallclock time after timekeeping subsystem is
resumed. This results in WALT accounting bugs.

Change-Id: I9c3b2fb9ffbf1103d1bd78778882450560dac09f
Signed-off-by: Pavankumar Kondeti <pkondeti@codeaurora.org>
[clingutla@codeaurora.org: Resolved trivial merge conflicts.]
Signed-off-by: Lingutla Chandrasekhar <clingutla@codeaurora.org>